A dialysis filter consists substantially of hollow fibers, i.e. cylindrical fibers, which traverse a housing longitudinally stretched. Thereby, the walls of the hollow fibers work due to semipermeable structures as membranes. At their ends, the hollow fibers are embedded in a casting compound. In the dialysis filter, the hollow fibers can be combined into modules with several square meter filter surface. In the dialysis by tangential flow filtration, also known as cross-flow filtration, blood/plasma is supplied to the hollow fibers by a first fluid circulation, which flows through them lengthwise. By a second fluid circulation, the dialysate is supplied usually by the countercurrent principle, but if possible also parallel to the blood stream. The housing thus has four ports, namely for each fluid stream two, one for supply and removal, respectively. On the inside of the hollow fiber membrane is thus the blood stream, and on the outside is the dialysate.
Another purification mechanism is convection. Here a pressure gradient across the semipermeable membrane is generated, whereby the fluid to be purified is pressed over the semipermeable membrane. Thereby, the substances are washed away in its current concentration. This purification process is not dependent on the concentration of substances in the purification solution, decisive are hereby only the concentration in the fluid to be purified, and the membrane properties, such as sieving coefficient, permeability etc. It is therefore of interest to know the filter properties at the beginning of the treatment as well as during the treatment.
A specific area of the filtration is the extracorporeal blood treatment for chronic or acute renal failure. Here, the fluid to be purified is the blood of the patient and the purification solution is the dialysate. In this specific case of the TFF it is decisive to replace in the treatments (in the chronic case typically three times per week) the blood purification function of the kidney as effective as possible. To ensure this, the Kt/V value has been established as measure of the treatment quality. The Kt/V is a parameter to determine the dialysis effectiveness and a key element for the evaluation of the dialysis efficiency. K is the clearance, which is determined by the urea content of the blood before and after the dialysis. The value t shows the effective dialysis time in minutes and V is the urea distribution volume. This refers to the content of water in the body which represents around 60% of the body mass. The aim of a treatment is to achieve a Kt/V≧1.2. In a normal treatment process values are achieved, which meet this criterion in general. However, adversities can occur during treatment, which affect negatively the treatment process as well as the treatment result. It is therefore important to monitor and control the influencing parameters during a treatment in order to be able to react to such adversities in a fast and especially targeted manner and to adjust the system parameters during the dialysis accordingly.
In modern dialyzers purification is achieved by use of convection due to the principle of the ultrafiltration. Hereby, not only substances to be removed or uremic substances are removed from the blood. Due to the applied pressure gradient (transmembrane pressure) across the filter membrane by the ultrafiltration pump (UF pump), plasma is convectively removed from the blood via the membrane. For this purpose, on the dialysate side from a closed system, a defined amount of dialysis solution is removed. Thereby, in the closed system, a negative pressure is generated which ensures that the same amount of blood passes over the semipermeable membrane to the dialysate side.
The loss of plasma volume must be compensated by supplying substituate solution. The substituate solution is typically an electrolytic solution. The ultrafiltration rate (UF-rate) describes the volume of the blood plasma deprive in such a manner per time unit and thus also the volume of substituate solution which must be fed back into the blood. The admixture of the substituate is performed either before the dialyzer (predilution) or after the dialyzer (postdilution). The upper limit for the substituate solution is typically 25-30% of the blood flow in the postdilution for a hemodiafiltration (HDF). For the predilution mode this limitation does not exist.
The substitution thus continuously compensates the fluid removed via the liquid deprivation by the ultrafiltration and avoids thereby volume losses. Thereby the substitution rate is defined by a specific volume per time unit.
The spent dialysis solution is replaced respectively by an equal volume of fresh dialysis solution. This can be done in a so-called balance chamber. Modern systems achieve here a maximum deviation of 0.07% during a several-hour dialysis session. To prevent mixing of the spent dialysis solution with the fresh dialysis solution, the two chambers are separated by a rubber membrane from each other.
A decisive process is the interaction of the filter membrane with blood. By this interaction the flow properties of the filter deteriorate both in transmembrane direction and in blood flow direction. These changes are caused for example by thrombocyte attachment on the membrane, clotting, chemical binding of blood components to the membrane or simply mechanical (flow-induced) pressing of the blood components on and even into the membrane.
Transmembrane direction or transmembranous herein refers to a flow of the blood over the membrane of the dialyzer or dialysis filter.
During clotting a gelatinous aggregation of red blood cells (erythrocytes) stabilized by fibrin filaments is formed. Unlike the term thrombus a coagulum describes a blood clot, which is located outside of a blood or lymph vessel (extravascular) and not inside (intravascular).
These and other changes of the system properties have various effects on the treatment process and the treatment quality. Especially, the treatment by hemodiafiltration is affected thereby, because here it is focused on the convective substance transport of medium molecular substances. The flow and stream properties changed especially by the deposits on the filter membrane lead to a changed demand for dialysis solution. By deterioration of the transmembrane flow properties or the permeability, also the sieving coefficient for uremic substances in the medium molecular weight range deteriorates, which has the result that by the same amount of convectively filtered fluid less uremic substances are removed from the blood circulation. Another effect is the reduction of the effective flow area, both in blood flow direction and in transmembrane direction. This results in a reduction of the active filter surface whereby it can lead to a deterioration of the diffusive purification. With new filters there is usually a buffering potential that is larger than the maximum physiologically filtering. Thereby, a reduction of the effective flow area can be limited to a certain degree. However, if this potential is exhausted, it leads to the above-described effect.
As suitable counteractions, or reactions to such changes, rinsing with saline for “cleaning” of the dialyzer, the addition of heparin to prevent further clotting or lowering of the ultrafiltration rate (UF) in order to reduce the hemoconcentration are generally accepted. The permeability of the membranes is determined by measuring the fluid volume, which passes at a given pressure difference at a temperature of 37° C. through a predetermined membrane surface the membrane and which is normalized for general comparability in terms of area unit, time unit, and pressure unit. As fluid for determining the ultrafiltration rate water is used.